Dynamic reflector array and method of making the same

ABSTRACT

A dynamic micro-structured reflector includes a substrate having a generally planar major surface and a plurality of cavities on the planar major surface. Each cavity having at least a first and second sidewalls set at an angle offset from the planar major surface. The first sidewall being a stationary optical face and the second sidewall being a dynamic optical face. The dynamic optical face is deflectable between a first position and a second position. The dynamic optical face in the first position redirects more light back to a light source than the dynamic optical face in the second position. Methods of making a dynamic micro-structured reflector are also disclosed.

The present application is a Divisional of U.S. patent application Ser.No. 10/716,341 filed Nov. 18, 2003 now U.S. Pat. No. 7,038,824.

FIELD OF THE INVENTION

The present invention relates generally to the field of dynamicreflectors. More specifically, the present invention pertains tomicro-structured dynamic optical reflectors.

BACKGROUND OF THE INVENTION

Retroreflectors are known devices which are often used as transponders.Retroreflectors receive light and reflect the light back in thedirection from whence it came. A passive retroreflector returns lightwith the same general characteristics of the incident light, preferablywith relatively high directional gain and relatively little spreading ofthe incident light beam. In contrast, an active retroreflector mayinclude an emitting device that can generate a user definable signal inresponse to the incident radiation beam.

Retroreflectors have found a wide variety of applications in numerousfields including communications systems, monitoring systems, andin-flight refueling systems. Examples of different types ofretroreflector structures include a corner-cube reflector, a hornreflector, a parabolic dish reflector, and a parabolic cylinderreflector.

Another application of retroreflectors is in the task of identifyingfriend-or-foe (IFF) in a battleground setting. Since the evolution ofweaponry which allowed opposing forces to fight through the exchange ofthe instrumentalities of war at a distance, fratricide killing has beena problem. IFF tasks are a delicate comprise between secure, ambiguousidentification and the maintenance of stealth positions. In typical IFFsystems, a radio or microwave frequency request is made by aninterrogation unit such as a plane or tank and a corresponding signal isreturned by the targeted unit. This is normally achieved by atransponder on the targeted unit that emits a coded return signal whenthe interrogation request is received. Other systems merely re-radiateor reflect the incident interrogation request, while some systemsmodulate the re-radiated or reflected signal in an distinctive manner.The interrogation unit then deciphers the received signal to determineif the targeted unit is a friend or foe. However, by emitting (i.e.,reflecting) a broadly directed response that is designed to have asufficient strength to reach the interrogation unit, some of theradiation may be detected by other units of the opposing force which mayreveal the position of the targeted unit.

SUMMARY OF THE INVENTION

Generally, the present invention pertains to micro-structured dynamicoptical reflectors.

In one illustrative embodiment, a dynamic micro-structured reflectorincludes a substrate having a generally planar major surface and aplurality of cavities on the planar major surface. Each cavity has atleast a first and second sidewalls set at an angle offset from theplanar major surface. The first sidewall is a stationary optical faceand the second sidewall is a dynamic optical face. The dynamic opticalface is deflectable between a first position and a second position. Thedynamic optical face in the first position redirects more light back toa light source than the dynamic optical face in the second position.

In another illustrative embodiment, a dynamic micro-structured reflectorincludes a plurality of cube-corner elements forming a cube-cornerarray. Each cube-corner element has two stationary optical faces and onedynamic optical face. The dynamic optical face is deflectable between afirst position and a second position. The dynamic optical face in thefirst position redirects more light back to a light source than thedynamic optical face in the second position.

In a further illustrative embodiment, a method of making a dynamicmicro-structured reflector includes the steps of: depositing a firstconducting layer on a substrate having a generally planar major surfaceand a plurality of cavities on the planar major surface, each cavityhaving at least a first and second sidewalls set at an angle offset fromthe planar major surface; patterning the first conducting layer to forma lower electrode on each first sidewall and adjacent portion of themajor planar surface; depositing a first dielectric layer on the lowerelectrode and the substrate; depositing and patterning a sacrificiallayer on the first sidewall first dielectric layer; depositing a seconddielectric layer on the sacrificial layer and first dielectric layer;depositing a second conducting layer on the second dielectric layer;patterning the second conducting layer to form an upper electrode overeach lower electrode; depositing a third dielectric layer on the upperelectrode and the second dielectric layer; depositing a reflecting layeron the first and second sidewall third dielectric layer; forming a holethrough the second and third dielectric layers to expose a portion ofthe sacrificial layer; and removing the sacrificial layer to form adynamic optical face being deflectable between a first position and asecond position and a portion of the dynamic optical face being spacedaway from the first dielectric layer a first distance in secondposition. The dynamic optical face in the first position redirects morelight back to a light source than the dynamic optical face in the secondposition.

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The Figures, Detailed Description and Examples which followmore particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a top view of an array of cube-corner elements according to anembodiment of the invention;

FIG. 2 is a top view of a single cube-corner reflecting element shown inFIG. 1;

FIG. 3A is a cross-sectional side view of FIG. 2 across line 3—3 in an“ON” state;

FIG. 3B is a cross-sectional side view of FIG. 2 across line 3—3 in an“OFF” state;

FIG. 4 is a top view of an array of cube-corner elements according to anembodiment of the invention;

FIG. 5 is a top view of a single cube-corner reflecting element shown inFIG. 4;

FIG. 6A is a cross-sectional side view of FIG. 5 across line 6—6 in an“ON” state;

FIG. 6B is a cross-sectional side view of FIG. 5 across line 6—6 in an“OFF” state;

FIG. 7A is a cross-sectional side view of dynamic micro-structuredreflector at a stage of fabrication where a sacrificial layer has beendeposited on a lower electrode;

FIG. 7B is a cross-sectional side view of dynamic micro-structuredreflector at a stage of fabrication where a top electrode has beendeposited;

FIG. 7C is a cross-sectional side view of dynamic micro-structuredreflector at a stage of fabrication where contact pads have beendeposited;

FIG. 7D is a cross-sectional side view of dynamic micro-structuredreflector at a stage of fabrication where a reflective layer has beendeposited and a via has been formed to the sacrificial layer;

FIG. 7E is a cross-sectional side view of dynamic micro-structuredreflector at a stage of fabrication where the sacrificial layer has beenremoved and in the “OFF” state;

FIG. 7F is a cross-sectional side view of dynamic micro-structuredreflector in the “ON” state.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description should be read with reference to the drawings,in which like elements in different drawings are numbered in likefashion. The drawings, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope of theinvention. Although examples of construction, dimensions, and materialsare illustrated for the various elements, those skilled in the art willrecognize that many of the examples provided have suitable alternativesthat may be utilized.

Generally, the present invention pertains to dynamic reflectors such as,micro-structured dynamic optical reflectors. While the present inventionis not so limited, an appreciation of various aspects of the inventionwill be gained through a discussion of the examples provided below.

For the following defined terms, these definitions shall be applied,unless a different definition is given in the claims or elsewhere inthis specification.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. Thus, for example, reference to a “scheduleparameter” includes a two or more schedule parameters. As used in thisspecification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

FIG. 1 is a top view of an array 100 of cube-corner elements 110according to an embodiment of the invention. The cube-corner elements110 can be disposed on a substrate 120 having a planar major surface.The cube-corner elements 110 form cavities on substrate 120 major planarsurface. In the illustrative embodiment, each element 110 is in theshape of a tetrahedral prism that has three exposed optical faces 108and an inverted apex 104. The optical faces of the cube-corner elementsdefine a micro-structured surface. Each element 110 can have astationary optical face 140 and a dynamic optical face 150. The array100 is configured to retro-reflect light incident light. The array 100can modify the amount of incident light that is retro-reflected in adynamic manner as described below.

In some embodiments, the substrate 120 can include electronics 130 tocontrol the position of some or all of the dynamic optical faces 150.The dynamic optical faces 150 can be controlled in unison or separatelyas desired. The dynamic optical face 150 can be deflectable between afirst position and a second position. In this embodiment, the dynamicoptical face 150 in the first (e.g., low) position redirects moreoptical light back to a light source than the dynamic optical face 150in the second (e.g., upper) position. As used herein, the first positioncan be a position that redirects more light back to the light source orthe second position can be a position that redirects more light back tothe light source. The dynamic optical face 150 actuates from a firstposition to a second position or from a second position to a firstposition. In either case the dynamic optical face actuates to or from aposition that redirects more light back to the light source.

FIG. 2 is a top view of a single cube-corner reflecting element 210shown in FIG. 1. In the illustrative embodiment, each element 210 is inthe shape of a tetrahedral prism that has three exposed optical faces240A, 240B, 250 and an inverted apex 204. Each element 210 can have astationary optical face 240A, 240B and a dynamic optical face 250. Inthe illustrative embodiment, the element 210 can include electronics 230to control the position of the dynamic optical face 250.

FIG. 3 is a cross-sectional side view of the element 210 shown in FIG. 2across line 3—3 in an “ON” state 300. With regard to FIG. 3A and FIG.3B, the element 310 forms a cavity or depression on the substrate 320major planar surface 301. Two optical faces 340, 350 are shown meetingat an inverted apex 304. The element 310 has a stationary optical face340 and a dynamic optical face 350. In the illustrative embodiment, aportion of, or substantially the entire surface of the dynamic opticalface 350, is deflectable relative to the stationary optical face 340.

A reflective layer 360 can be disposed on the stationary optical face340 and the dynamic optical face 350. The reflective layer 360 can bespecularly reflective. The reflective layer 360 can be metallic.Suitable materials for use as reflective layer 360 can include anysuitable reflective material or composite material, for example,aluminum, gold, silver, tin and combinations thereof. In someembodiments, the reflective layer may be any suitable thickness such as,for example, between 100 to 1200 nm thick.

The element 310 has a depth “D” equal to the distance from the majorplanar surface 301 to the inverted apex 304. In some embodiments, depth“D” is between 10 microns and 100 microns. In other embodiment, depth“D” is between 30 microns to 50 microns. However, it is recognized thatdepth “D” may be any suitable dimension, depending on the application.

The dynamic optical face 350 has a length “L” equal to the distance fromthe major planar surface 301 to the inverted apex 304. In someembodiments, length “L” is between 10 microns to 100 microns. In otherembodiments, length “L” is between 30 microns to 50 microns. However, itis recognized that length “L” may be any suitable dimension, dependingon the application.

The dynamic optical face 350 can have a lower electrode 352 on, adjacentto, or in the substrate 320, extending along at least a portion of thelength “L” of the dynamic optical face 350. The dynamic optical face 350can have an upper electrode 354 adjacent to the lower electrode 352,which may also extend along, at least a portion of, the length “L” ofthe dynamic optical face 350. The upper electrode 354 can have a fixedend 358 and an opposing free end 356. In some embodiments, the fixed end358 can be affixed at or near the substrate 320 major planar surface301, and the free end 356 can be movable at or near the inverted apex304.

With regard to FIG. 3A, when voltage sufficient is applied across theupper electrode 354 and lower electrode 352, the free end 356 deflectsfrom a second “OFF” position to a first “ON” position under anelectrostatic force. FIG. 3A shows the free end 356 in the first “ON”position. The first “ON” position configures the dynamic optical face350 to cooperate with the stationary optical face 340 redirecting alarger amount of incident optical light 370 from a light source 302,into reflected light 380 directed in a path parallel to the incidentoptical light 370 path and back to the light source 302.

FIG. 3B is a cross-sectional side view of FIG. 2 across line 3—3 in an“OFF” state. When the voltage is removed across the upper electrode 354and lower electrode 352, the free end 356 deflects from a first “ON”position to a second “OFF” position. FIG. 3B shows the free end 356 inthe second “OFF” position. The second “OFF” position configures thedynamic optical face 350 to cooperate with the stationary optical face340 redirecting a large amount of incident optical light 370 from alight source 302, into reflected light 381 directed into pathsnon-parallel to the incident optical light 370 path and away from thelight source 302. The dynamic optical face 350 free end 356 deflectsaway from the inverted apex 304 creating an air gap 306 between the freeend 356 and the apex 304 and/or lower electrode 352. In someembodiments, the air gap 306 can be from 2 microns to 10 microns, or 2microns to 5 microns, but any suitable gap may be utilized.

FIG. 4 is a top view of an array 400 of cube-corner elements 410. Thecube-corner elements 410 can be disposed on a substrate 420 having aplanar major surface. In the illustrative embodiment, the cube-cornerelements 410 form cavities on substrate 420 major planar surface. Eachelement 410 is in the shape of a tetrahedral prism that has threeexposed optical faces 408 and an inverted apex 404. The optical faces ofthe cube-corner elements define a micro-structured surface. Each element410 can have a stationary optical face 440 and a dynamic optical face450. The array 400 is configured to retro-reflect incident light, andcan modify the amount of incident light that is retro-reflected in adynamic manner as described below.

In some embodiments, the substrate 420 can include electronics 430 thatcan control the position of the dynamic optical faces 450. The dynamicoptical faces 450 can be controlled in unison or operate separately, asdesired. As shown below, the dynamic optical face 450 can be deflectablebetween a first position (e.g., lower) and a second position (e.g.,upper), where the dynamic optical face 450 redirects more optical lightback to a light source in the first position than in the secondposition.

FIG. 5 is a top view of a single cube-corner reflecting element 510shown in FIG. 4. Each element 510 is in the shape of a tetrahedral prismthat has three exposed optical faces 540A, 540B, 550 and an invertedapex 504. In the illustrative embodiment, each element 510 can have astationary optical face 540A, 540B and a dynamic optical face 550. Insome embodiments, the element 510 can include electronics 530 to controlthe position of the dynamic optical face 550, but this is not requiredin all embodiments.

FIG. 6A is a cross-sectional side view of the element 510 shown in FIG.2 across line 6—6 in an “ON” state 600. With regard to FIG. 6A and FIG.6B, the element 610 forms a cavity or depression on the substrate 620major planar surface 601. Two optical faces 640, 650 are shown meetingat an inverted apex 604. The element 610 has a stationary optical face640 and a dynamic optical face 650. A portion of, or substantially theentire surface of the dynamic optical face 650 can be deflectablerelative to the stationary optical face 640.

A reflective layer 660 can be disposed, or otherwise provided on thestationary optical face 640 and dynamic optical face 650. The reflectivelayer 660 can be specularly reflective. The reflective layer 660 can bemetallic. Suitable materials for use as reflective layer 360 can includeany suitable reflective material or composite material, for example,aluminum, gold, silver, tin and combinations thereof. The reflectivelayer can be any suitable thickness such as, for example, between 500and 1200 nm thick.

The element 610 has a depth d equal to the distance from the majorplanar surface 601 to the inverted apex 604. In some embodiments, depth“D′” is between 10 microns and 100 microns. In other embodiments, depth“D′” is between 30 microns and 50 microns. However, it is recognizedthat depth “D′” may be any suitable dimension, depending on application.

The dynamic optical face 650 has a length “L′” equal to the distancefrom the major planar surface 601 to the inverted apex 604. In someembodiments, length “L′” is between 10 microns and 100 microns. In otherembodiments, length “L′” is between 30 microns and 50 microns. However,it is recognized that length “L′” may be any suitable dimension,depending on application.

The dynamic optical face 650 can have a lower electrode 652 on, adjacentto, or in the substrate 620, extending along at least a portion of thelength “L′” of the dynamic optical face 650. The dynamic optical face650 can have an upper electrode 654 adjacent to the lower electrode 652,which may also extend along at least a portion of the length “L′” of thedynamic optical face 650. The upper electrode 654 can have a fixed end658 and an opposing free end 656. In some embodiments, the fixed end 658can be affixed at or near the substrate 620 major planar surface 601,and the free end 656 can be movable at or near the inverted apex 604.

With regard to FIG. 6A, when sufficient voltage is applied across theupper electrode 654 and lower electrode 652, the free end 656 deflectsfrom a second “OFF” position to a first “ON” position under anelectrostatic force. FIG. 6A shows the free end 656 in the first “ON”position. The first “ON” position configures the dynamic optical face650 to cooperate with the stationary optical face 640 redirecting alarger amount of incident optical light 670 from a light source 602,into reflected light 680 directed in a path parallel to the incidentoptical light 670 path and back to the light source 602.

FIG. 6B is a cross-sectional side view of FIG. 5 across line 6—6 in an“OFF” state. When the voltage is removed across the upper electrode 654and lower electrode 652, the free end 656 deflects from a first “ON”position to a second “OFF” position. FIG. 6B shows the free end 656 inthe second “OFF” position. The second “OFF” position configures thedynamic optical face 650 to cooperate with the stationary optical face640 redirecting a large amount of incident optical light 670 from alight source 602, into reflected light 681 directed into pathsnon-parallel to the incident optical light 670 path and away from thelight source 602. The dynamic optical face 650 free end 656 deflectsaway from the inverted apex 604 creating an air gap 606 between the freeend 656 and the apex 604 and/or lower electrode 652. In someembodiments, the air gap 606 can be from 2 microns to 10 microns, or 2microns to 5 microns, but any suitable air gap can be used.

FIG. 7A is a cross-sectional side view of a dynamic micro-structuredreflector 700 at a stage of fabrication where a sacrificial layer 740has been deposited on a lower electrode 720, which has been deposited ona dielectric layer 710. The substrate 701 is a micro-structured articlethat can have a generally planar major surface 702 and a plurality ofcavities 703. Each cavity 703 can have at least a first 704 and second705 sidewall set at an angle offset from the planar major surface 702.In an illustrative embodiment, the substrate 701 has a prismaticmicro-structured surface, such as a cube-corner micro-structuredsurface. For example, an illustrative cube-corner micro-structuredsubstrate 701 is commercially available from the 3M Company (St. Paul,Minn.) under the tradename 3000X.

A first or lower conducting layer 720 can be deposited on the substrate701 or dielectric layer 710. In some embodiments, the substrate 701 canbe a dielectric material. First or lower conducting layer 720 can bepatterned to form a lower electrode 720 on the first sidewall 704 and aportion of the major planar surface 702. A dielectric layer 712 can thenbe deposited on the lower electrode 720. A sacrificial layer 740 can bedeposited and patterned on the dielectric layer 712 above the lowerelectrode 720 and the first sidewall 704. The sacrificial layer 740 willbe removed later in the processing, as described below.

The sacrificial layer 740 serves as a spacer between the lower electrode720 and the upper electrode 722 (to be formed) so that when it isremoved later in the process, the upper electrode 722 will be releasedand free to move. The sacrificial layer 740 can be any material, thancan be deposited, patterned, and later selectively removed with an etchthat does not attack the other films exposed during the etch. Examplesof sacrificial layers that can be used are aluminum, molybdenum,polyimide, silicon dioxide, and the like. The sacrificial layer 740 canbe any suitable thickness such as, for example, 250 to 1000 angstroms.

FIG. 7B is a cross-sectional side view of dynamic micro-structuredreflector 700 at a stage of fabrication where a top electrode 722 hasbeen deposited. A dielectric layer 716 can be deposited on the lowerelectrode 720 dielectric layer 712 and the sacrificial layer 740. Aconducting layer 722 then can be patterned to form an upper electrode722 on the first sidewall 704 and a portion of the major planar surface702 above the lower electrode 720. A dielectric layer 718 can bedeposited on the upper electrode 722. Hence, the lower electrode 720 andthe upper electrode 722 are encapsulated by dielectric material toelectrically isolate the lower electrode 720 from the upper electrode722.

The dielectric layers 710, 712, 716, 718 can be any suitableelectrically insulating material such as, for example, silicon nitride,silicon oxide, polyimide, and the like. The dielectric layers 710, 712,716, 718 can be any suitable thickness such as, for example, 0.5 to 1micron. The electrode layers 720, 722 can be any electrically conductingmaterial such as, for example, chrome, aluminum, gold, silver, and thelike. The electrode layers 720, 722 can be any suitable thickness suchas, for example, 500 to 1000 angstroms.

FIG. 7C is a cross-sectional side view of dynamic micro-structuredreflector 700 at a stage of fabrication where contact pads 721, 723 havebeen deposited. Vias are formed and contact pads 721, 723 are depositedinto the vias to form connections to the lower electrode 720 and theupper electrode 722. The contact pads 721, 723 can be any suitableconductive material such as gold. The contact pads 721, 723 can be anysuitable thickness such as, for example, 250 to 1000 angstroms of chromefollowed by 0.5 to 1 micron of gold.

FIG. 7D is a cross-sectional side view of dynamic micro-structuredreflector 700 at a stage of fabrication where a reflective layer 750 hasbeen deposited and a hole 760 has been formed to intersect thesacrificial layer 740. A reflecting layer 750 is deposited and patternedonto the top dielectric layer 718 on the first sidewall 704 and secondsidewall 705. The reflecting layer 750 can be specularly reflective asdescribed above. A hole 760 can be formed through the dielectric layers716, 718 over the sacrificial layer 740 to expose a portion of thesacrificial layer 740. The hole 760 provides an opening to remove thesacrificial layer 740.

FIG. 7E is a cross-sectional side view of dynamic micro-structuredreflector 700 at a stage of fabrication where the sacrificial layer 740has been removed and in the “OFF” state. The sacrificial layer 740 canbe selectively removed with a selective etch such as a wet etch or a dryplasma etch. Once the sacrificial layer 740 is removed, the upperelectrode 722 can be configured and arranged so that residual stress inthe upper electrode 722 structure will cause it to curve away from thelower electrode 720 when released. This is the “OFF” state of thedevice, where the upper electrode 722 is not allowing optical light toretro-reflect back to a light source.

FIG. 7F is a cross-sectional side view of dynamic micro-structuredreflector 700 in the “ON” state. Applying a voltage between the upperelectrode 722 and lower electrode 720 operates the dynamic reflector700, attracting the upper electrode 722 to the lower electrode 720 viaelectrostatic forces. The electrostatic force pulls the upper electrode722 against the lower electrode 720 (with dielectric material betweenthem) allowing the reflective surface 750 on the upper electrode 722 tocooperate with the reflective surface 750 on the second sidewall 705 toretro-reflect incident optical light back to the light source. The upperelectrode 722 can be modulated between open and closed positions.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention can be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the instant specification.

1. A method of making a dynamic micro-structured reflector comprisingthe steps of: depositing a first conducting layer on a substrate havinga generally planar major surface and a plurality of cavities on theplanar major surface, each cavity having at least a first and secondsidewalls set at an angle offset from the planar major surface,patterning the first conducting layer to form a lower electrode on eachfirst sidewall and adjacent portion of the major planar surface;depositing a first dielectric layer on the lower electrode and thesubstrate; depositing and patterning a sacrificial layer on the firstsidewall first dielectric layer; depositing a second dielectric layer onthe sacrificial layer and first dielectric layer; depositing a secondconducting layer on the second dielectric layer; patterning the secondconducting layer to form an upper electrode over each lower electrode;depositing a third dielectric layer on the upper electrode and thesecond dielectric layer; depositing a reflecting layer on the first andsecond sidewall third dielectric layer; forming a hole through thesecond and third dielectric layers to expose a portion of thesacrificial layer; and removing the sacrificial layer to form a dynamicoptical face being deflectable between a first position and a secondposition and a portion of the dynamic optical face being spaced awayfrom the first dielectric layer a first distance in the second position;wherein, the dynamic optical face in the first position redirects morelight back to a light source than the dynamic optical face in the secondposition.
 2. The method according to claim 1, further comprising thestep of applying a voltage between the lower electrode and upperelectrode reducing the first distance until the dynamic optical face isin the first position.
 3. The method according to claim 1, wherein thestep of removing the sacrificial layer to form a dynamic optical facebeing deflectable comprises removing the sacrificial layer to form adynamic optical face where substantially the entire dynamic optical facedeflects between the first position and the second position.
 4. Themethod according to claim 1, wherein the step of depositing a firstconducting layer on a substrate having a generally planar major surfaceand a plurality of cavities on the planar major surface comprisesdepositing a first conducting layer on a substrate having a generallyplanar major surface and a plurality of cavities, each cavity having adepth of 10 microns to 100 microns.
 5. The method according to claim 1,wherein the step of depositing a first conducting layer on a substratehaving a generally planar major surface and a plurality of cavities onthe planar major surface comprises depositing a first conducting layeron a substrate having a generally planar major surface and a pluralityof cavities, each cavity having a depth of 30 microns to 50 microns. 6.The method according to claim 1, wherein the step of depositing a firstconducting layer comprises depositing a first metal layer a thickness of500 to 1000 angstroms.
 7. The method according to claim 1, wherein thestep of depositing a second conducting layer comprises depositing asecond metal layer a thickness of 500 to 1000 angstroms.
 8. The methodaccording to claim 1, wherein the step of depositing a sacrificial layercomprises depositing a sacrificial layer a thickness of 250 to 1000angstroms.
 9. The method according to claim 1, wherein the step ofdepositing a first conducting layer on a substrate having a generallyplanar major surface and a plurality of cavities on the planar majorsurface comprises depositing a first conducting layer on a substratehaving a generally planar major surface and a plurality of cube-cornerstructures in the planar major surface.
 10. The method according toclaim 1 further comprising the step of depositing an electricallyinsulating layer on the substrate before depositing the first conductinglayer, and then depositing the first conducting layer on the insulatinglayer.